1. Technical Field
The present invention relates to an apparatus for switching time division multiplex channels from a plurality of input data streams to a plurality of output data streams. Such apparatuses may be used in switched networks, for instance in telephone systems.
2. Background Art
Modern circuit switched networks are required to handle multiple TDM (Time Division Multiplex) data streams having different stream data rates and/or bit-channel offsets.
The stream data rate employed for TDM is 1.024 Mb/s (million bits/second) and multiples/sub-multiples thereof, i.e. 2.048 Mb/s, etc. currently reaching speeds of upto 32.768 Mb/s. The streams are divided up into a 125 μs frames which are further divided into channels, each consisting of 8 bits (binary digits). The number of channels varies in direct proportion with the stream rate, as exemplified in Table 1.
TABLE 1Stream Rate (Mb/s) vs. Channels/FrameStream Rate (Mb/s)Channels/Frame 1.024 16 2.048 32 4.096 64 8.19212816.38425632.768512
The stream data arriving at a TDM switch on any particular stream is often offset from the nominal position of the start of channel 0 aligned to the local frame boundary. This may be for many reasons such as non-standard frame boundary signal definition within the system, system delays, etc. The range of offsets possible, usually expressed in ¼ bit increments, depends on the stream data rate, as exemplified in Table 2. A fraction bit offset is referred to as a ‘step offset’ herein. Thus, for example, a stream having an offset of 2 channels and 4¾ bits is described as having a channel offset of 2 channels, a bit offset of 4 bits and a step offset of ¾ of a bit.
TABLE 2Stream Rate (Mb/s) vs. Bit/Channels OffsetStream Rate (Mb/s)Bit-channel offset 1.024 0–15 channels, 0–7¾ bits 2.048 0–31 channels, 0–7¾ bits 4.096 0–63 channels, 0–7¾ bits 8.1920–127 channels, 0–7¾ bits16.3840–255 channels, 0–7¾ bits32.7680–511 channels, 0–7¾ bits
In TDM systems, channel data is repeated every frame at 125 μs intervals and will typically carry the information associated with one telephone call, although this is just as likely to be data traffic. Channels are allocated numbers and the convention is to number these from 0 (zero), such that, for example, a 1.024 Mb/s TDM data stream contains frames which have 16 channels numbered 0 to 15 as shown in FIG. 1.
In circuit switched networks, switching involves the transfer of channel data from a set of input streams (SN1) to a set of output streams (SN0). This process often involves input and output streams having different stream rates and input stream bit-channel offsets.
FIG. 2 shows a frame which illustrates TDM switching between channel 14 of a 1.024 Mb/s input stream having a channel offset of 5 channels with respect to the frame boundary reference and channel 19 of a 2.048 Mb/s output stream. Using current techniques, this involves a multi-stage process of generating intermediate streams having the same stream rate as the output stream. Known devices offer limited configurability, i.e. all input streams must have a common data rate as do all output streams.
Conventionally, the switching of channel data from TDM data streams having different rates involves a two step process:
1. First, the source stream, stream rate=RS, is converted to the same stream rate as the destination stream, stream rate, RD. If RS>RD, then the source stream is converted into n streams at rate RD, with n being equal to the ratio RS/RD in order to maintain the total number of channels. For example, if RS=16.384 Mb/s (256 channels/frame) and RD=2.048 Mb/s (32 channels/frames), then the source stream is converted into n=8 streams with rate RD=2.048 Mb/s, i.e. 32×8=256. If RS<RD, then the source stream is converted into a single stream at rate RD which may either be combined with other stream(s) having rate RS or, alternatively, the unused channels in the derived stream padded with other data. The second step involves simple channel switching between the stream derived in Step 1 and the destination stream which now have the same stream rate.